HVAC · Thermodynamics · Problem 13PDFSolution in PDF ↓
HVAC · Thermodynamics · Problem 13
Problem & Solution
Video Synthesis
Problem: Steam with a quality of 90% expands icing tropically from 350 psi a to 120 psi a.
Given: 350 psi a to 120 psi a; 350 psi a; 120 psi a, but we don't know anything else
Approach: This is a really good problem for learning how to use the steam tables, making sure you're able to find the right values and br...
Calc: We'll call it state 1 and the pressure at state 1 is 350 psi a.
Calc: At state 2, we also know the pressure.
Result: And the best answer choice is B.
Office Hours
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Student questions asked in live office hours about this problem
OH 115: HVAC: Thermo #13
Q: Why can you look up values for steam using just pressure and quality, when normally you need two independent properties to define a state?
A: When quality is between 0 and 1, you're in the two-phase region where pressure and temperature are linked on the saturation line. Quality then pinpoints your exact location on that line, giving you all other properties. The calculation uses quality = (s1 - sf) / sfg, so knowing pressure/temperature and quality gives you the two pieces of information needed to solve for enthalpy, entropy, and specific volume.
OH 32: HVAC_THERMO-13
Q: Can you show where the star point is on an enthalpy-pressure diagram and how to find enthalpy and entropy values for this constant entropy process?
A: This problem isn't suited for graphical solution—use steam tables instead. If you want to visualize it, check out a mollier diagram (which works better for constant enthalpy or entropy processes), but for the PE exam, stick with steam tables and reference both the video and written solutions for clarity.
OH 82: HVAC: Thermo Module #13
Q: Why spend 8 minutes using steam tables and interpolation when the Mollier diagram can solve the same problem much faster?
A: Both methods are valuable—the Mollier diagram is fast, but mastering steam tables teaches you the underlying process that might be required for differently-formatted problems. During practice, take time to explore both approaches; during the exam, use the fastest method you're confident in, then revisit flagged problems if time permits to build additional certainty.
OH 88: HVAC: Thermo Module #13
Q: All the pressures in problem thermo 13 don't have values in the steam tables within handbook version 1.9—what's the alternative?
A: The values are definitely in the steam tables; you just need to use the pressure-organized version and interpolate. For this problem with 350 psi initial and 120 psi final pressure, find H1 and S1 by interpolation, apply the isentropic condition (S2=S1), determine quality at state 2, then calculate H2 and ΔH.